Riverside Hoverflies

Eristalis nemorum DSC_3087

If you seem male hoverflies buzzing around above a female on a  flower,  Ball & Morris tells us that you’re spying on the courting rituals of Eristalis nemorum. They were getting quite serious, dive-bombing and bumping her but she paid no heed to  the boys showng off.

E. nemorum is referred to by Falk as the Stripe-faced Drone Fly – presumably to help us remember a key difference between it and its near relative Eristalis arbustorum, the Plain-faced Dronefly. For some reason, many hoverflies have common names in Dutch but not in English. Arbustorum, for instance, is the kleine bijvlieg (small fly-by?). Look at the relevant EOL page and you’ll also learn that in Norway it has a similar name, the lita dronefluge.  The Finns always go their own way, its the Pihasurri. That certainly baffled Google Translate!

We saw other Eristalis hoverflies on the same August riverside walk, but they are so hard to tell apart unless you have good shots of their happy faces. Supposedly you can distinguish E. nemorum and arbustorum by the coloured cells on their wing margins (Stigma) but its not easy. If pushed, based on Falk’s galleries, I’d guess that the First of the two below is a nemorum female and the second an arbustorum female.

Eristalis nemorum f DSC_3050 - CopyEristalis arbustorum m. DSC_3107

I found an interesting explanation of the variations in colour you see in these flies here. Naturalist Davina White points to a a study which says that “the yellow/orange patches in adults are larger when the temperature was higher in the larval phase. So, the adults emerging in the summer are brighter than the adults emerging in the spring. It is thought that this variation in appearance has a function. Two theories exist: 1) The brighter adults can handle the higher temperatures of the summer better than darker adults, by not overheating. Conversely, during spring, darker adults heat up faster. 2) As these flies mimic bees, and bees themselves display phenotypic plasticity, it would make sense for the flies to mirror this behaviour as well. Also, darker adults mimic solitary bees, which are more common during spring and lighter adults mimic wasps and lighter bees, which are more abundant during summer. Of course, the theories are not mutually exclusive. The idea behind both theories is that the flies are more successful (live longer and/or have more offspring) in their respective seasons.” So now you know!





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